1. Field of the Invention
This invention relates to a method for the manufacture of a copper alloys containing chromium and tin, which are advantageously useful as a lead frame material for integrated circuits (IC's).
2. Description of Prior Art
In recent years, growing integration of semiconductor circuits, emergence of IC's which are high in current capacity, and the widespread trend toward minimization in of lead frames, in due respect to preservation of natural resources have all gone to presenting ever exacting demands on the quality of copper alloys used as the material for the lead frames e.g., in terms of strength, workability, electroconductivity, and heat resistance. To be specific, a material is rated as optimum when it wholly satisfies the requirements that its tensile strength exceeds 50 kg/mm.sup.2, that elongation at breaking should exceed 7%, that electroconductivity exceeds 80% I.A.C.S., and so on.
Heretofore, CDA (Copper Development Association in the U.S.A.) alloy 194, phosphor bronze, tin-containing heat-resistant copper, copper-chromium alloys, copper-chromium-tin alloys, etc., have been available as alloy materials for lead frames. Of these alloy materials, however, CDA alloy 194, phosphor bronze, and tin-containing heat-resistant copper do not wholly satisfy the requirements mentioned above.
CDA alloy 194 displays 45 kg/mm.sup.2 of tensile strength, 4% of elongation at breaking, and 65% I.A.C.S. of electroconductivity, and thus fails to satisfy the three noted characteristics. Phosphor bronze displays 55 kg/mm.sup.2, 8%, and 15% I.A.C.S., respectively, of the aforementioned properties, thus exhibiting insufficient electronconductivity while satisfying the characteristics of strength and elongation. Tin-containing heat-resistant copper displays 38 kg/mm.sup.2, 4%, and 84% I.A.C.S. respectively of the aforementioned properties, thus exhibiting satisfactory electroconductivity but failing to satisfy the other characteristics of strength and elongation.
It has been heretofore disclosed in Japanese Patent Laid Open Publication No. 123923/1977 that a copper-chromium alloy containing not more than about 5% by weight of chromium and an improved type thereof, i.e., copper-chromium-tin alloy containing not more than 1% by weight of chromium and between 0.30 and 0.95% by weight of which are produced by adding chromium, the element capable of forming a solid solution with copper at elevated temperatures and practically incapable of forming such a solid solution at low temperatures, subjecting the alloys to a solid solution treatment at elevated temperatures and then quenching the solid solution, thereby allowing the added elements to precipitate on aging in the copper matrixes, satisfy all the aforementioned requirements. But to obtain a copper-chromium alloy or a copper-chromium-tin alloy which satisfies these characteristics it was necessary that the cast alloy mass mentioned above should be subjected, either directly or after a hot working step, to a solid solution treatment generally at amply high temperatures of about 1000.degree. C., then quenching the resultant alloy mass, and further subjecting the quenched alloy mass to an age-hardening treatment. When the solid solution treatment is carried out in air, it entails vigorous oxidation of the interior as well as the surface of the alloy mass. This treatment, therefore, must be carried out under an atmosphere of a non-oxidizing gas and, consequently, suffers from increased production costs. Furthermore, when the large alloy mass is subjected to the solid solution treatment, it becomes extremely difficult to produce a homogeneous alloy satisfying all the characteristics mentioned above because the quenching treatment to be given subsequently to the solid solution treatment is not capable of conferring an ample quenched effect uniformly upon the entire mass.
To cope with this situation, the present inventors perfected an invention titled "Conductive Alloy for Integrated Circuits and Method for Manufacture Thereof" see (Japanese Patent Laid Open Publication No. 65956/1981) to provide improvements in and concerning the aforementioned method for manufacture of copper-chromium alloy and copper-chromium-tin alloy. Specifically, this method produces an eutectic dispersion-strengthened alloy solely by the steps of melting a copper-chromium alloy with addition of other elements such as tin, chill-cast material of a structure having a uniformly dispersed phase of fine chromium particles, and subjecting the cast material, without being intervened by any solid solution treatment and aging treatment, to cold working and annealing. But when a cast alloy mass of a large volume is to be produced by this chill casting method, it encounters a difficulty in the industry to attain a cooling speed enough to impart uniformly to the cast alloy mass the aforementioned dispersed eutectic structure. For the production of a material which must be available as an inexpensive mass-producible article such as a lead frame, this method has not found acceptance.
After all, the copper-chromium alloy and copper-chromium-tin alloy inevitably suffer from a high production cost and barely find utility in small-scale manufacture of the nature of trial production or experimental production because the aforementioned known method of manufacture brings about an increase in cost and the material is subject to a rigid restriction that it should be amply small in size.